Recently, the High Pressure Acid Leach (HPAL) using sulfuric acid has been gathering attention, as a hydrometallurgical process for nickel oxide ore. This process does not comprise dry steps, such as a drying step and a roasting step or the like, but comprises a continuous wet process. Thus, it is advantageous in regards to energy and cost. Additionally, this process has, at the same time, the advantage of capable of providing a mixed nickel-cobalt sulfide whose nickel grade is upgraded up to about 50% to 60% by weight or so.
As exemplified, for example in FIG. 5, the HPAL process for producing the mixed nickel-cobalt sulfide comprises a pretreatment step (1) of cracking and classifying nickel oxide ore into slurry (hereinafter referred to as “ore slurry”); a leaching step (2) of producing leached slurry, while stirring the ore slurry under temperature of 220 to 280 degrees C. by adding sulfuric acid to the produced ore slurry; a solid-liquid separation step (3) of solid-liquid separation the leached slurry to produce leachate containing nickel and cobalt (crude nickel sulfate aqueous solution) and a leached residue; a neutralization step (4) of neutralizing impurities by adding neutralizer (for example, calcium carbonate) to the produced crude nickel sulfate aqueous solution; a dezincification step (5) of removing zinc, as a zinc sulfide, by adding a hydrogen sulfide gas to a neutralized crude nickel sulfate aqueous solution; a sulfidization step (6) of producing a mixed nickel-cobalt sulfide and a nickel barren solution by adding the hydrogen sulfide gas to a produced dezinced final solution; and a detoxification step (7) of detoxifying the leached residue produced in the solid-liquid separation step (3), the zinc sulfide produced in the dezincification step (5), and the nickel barren solution and the like produced in the sulfidization step (6) (for example, see Patent Document 1).
In the hydrometallurgical process, because the more the amount of ore slurry to be treated in the steps increases, the larger the treatment facility becomes, it will become necessary to reduce the amount of ore slurry, that is, to adjust the ore slurry at a high concentration. On the other hand, it will become necessary to lower the viscosity of the ore slurry, that is, to prepare the ore slurry at a low concentration in order to facilitate a subsequent pipeline transfer.
Thus, in the above-mentioned hydrometallurgical process, there is a conflicting request to reduce the facility cost and facilitate a transfer of the ore slurry. To this end, it has been conventionally forced to proceed a treatment by adjusting an ore slurry concentration to 10% by weight or so.
In this connection, the hydrometallurgical process, such as the High Pressure Acid Leach (HPAL) process or the like are implemented by the plant facility comprised of a boiler generating steam for reaction temperature control of each step, a hydro sulfide production facility producing a hydro sulfide gas used mainly in the above-mentioned steps (5) and (6), further an irrigation facility, a power facility, and a plant facility composed of pipes, such as a flow feeding pipe sequentially coupling the each step, in addition to facilities responsible for the above-mentioned each step.
However, there remains unsettled a problem in this practical plant given in the following. That is, nickel oxide ore to be treated is transported as slurry among each step, but the slurry notably accelerates deterioration of the facility materials by wear. Among other things, a repair frequency is high especially in the facilities, such as a pipe and a pomp or the like used in the leaching step (2). This is the major cause of an increase in the maintenance cost and a decrease in an operation rate. Particularly, since chromite contained in the nickel oxide ore has a large particle diameter and is solid, the chromite is ingredients noticeably accelerating deterioration of the pipe and the pomp or the like by wear in the hydrometallurgical plant involving the transportation of the slurry. Thus, it is desirable to remove the chromite from crude ore to be treated in the leaching step (2).
As a countermeasure, there has been proposed in Patent Document 2 a technique of recovering chromite from ore slurry. Specifically, the technique disclosed in Patent Document 2 is a technique in which the chromite is physically separated depending on a particle diameter, with the ore slurry as a starting material, and recovers particles containing the chromite after a chromite particle and other particles are purified and separated. The technique, as shown in FIG. 6, takes advantage of a property that a particle diameter of the chromite in the ore slurry shows a relatively large grain distribution compared with that of the other particles, and thus their difference is noticeable, inter alia, in the vicinity of 50 to 100 μm.
However, this technique can only provide a recovery rate of 30% or so, as a recovery rate of target classification slurry containing the chromite, when the ore slurry is treated, as mentioned above, by adjusting an ore slurry concentration to 10% by weight or so. Namely, there still remains unsettled a problem that particles of 70% or so which have a particle size to be recovered and are contained in the ore slurry is remained without being recovered therefrom. Incidentally, the recovery rate of the target classification slurry indicates that which is calculated by an equation of “particle weight [g] more than target classification point in recovered slurry/particle weight [g] more than target classification point in supplied slurry”.
In the conventional physical separation method, it has been taken measures of improving a recovery rate of the target classification slurry normally by reducing a slurry concentration to be supplied to 5% by weight, after diluting the ore slurry of 10% by weight, in order to improve a recovery rate of the target classification slurry. In this instance, it however entails shortcomings that it needs to provide a large facility attendant to the dilution of the ore slurry. Furthermore, in this case, it is necessary for the ore slurry once diluted to concentrate, and therefore the conventional technique is unfavorable as a real operation.
Since the chromite contained in the ore slurry can be served that is raw materials for metallic chromium and a chromium compound, it is expected to be reusing thereof. Nonetheless, as mentioned above, it has been considered to one of the main factors of causing serious wear in the ore slurry transporting facility (pipe and pomp, or the like) extending to the leaching step (2) as specific gravity and hardness of the chromite are greater than those of the other particles.
Accordingly, it has been longing hitherto for the advent of a method able to improve a recovery rate of the chromite, even if an ore slurry concentration is adjusted to 10% by weight or so. In particular, it has been wanted the emergence of a method able to efficiently recover the chromite from the ore slurry produced by treating the nickel oxide ore that is raw materials.